Method and apparatus for digital subscriber line access multiplexer stacking

ABSTRACT

An architecture for stacking DSLAMs employs a master DSLAM having a control circuit and an uplink communication circuit to a central office and two LVDS circuits for communication with two DSLAM boxes in adjacent circuit interconnection to the master DSLAM. Multiple slave DSLAMs each having a programmable control circuit and two LVDS circuits for communication with two DSLAM boxes immediately adjacent on either side in the circuit are connected in a ring or split stack arrangement. A control circuit in the master selectively programs the control circuits in each slave DSLAM to control the direction of communication in the LVDS circuits as round robin in a first selected mode and a split in a second selected mode to accommodate the desired structure.

FIELD OF THE INVENTION

This invention relates generally to the field of Digital Subscriber LineAccess Multiplexers (DSLAMs) and, more particularly, to configurationand interconnection of DSLAMs to allow stacking efficiently with faulttolerance redundancy at low cost.

BACKGROUND OF THE INVENTION

Digital Subscriber Line Access Multiplexers are required forcommunications between digital subscriber line (DSL) users and thecentral office (CO). Typically, the “pizza box” sized DSLAMs are stackedat remote sites to provide higher density and ease of interconnection.Conventional stacking configurations of the DSLAMs are shown in FIGS. 1a and 1 b of the drawings based on single link daisy chain and starrespectively.

SUMMARY OF THE INVENTION

An architecture for stacking DSLAMs according to the present inventionemploys a master DSLAM having a control circuit and an uplinkcommunication circuit to a central office and two LVDS circuits forcommunication with two DSLAM boxes in adjacent circuit interconnectionto the master DSLAM. Multiple slave DSLAMs each having a programmablecontrol circuit and two LVDS circuits for communication with two DSLAMboxes immediately adjacent on either side in the circuit are connectedin a ring or split stack arrangement. A control circuit in the masterselectively programs the control circuits in each slave DSLAM to controlthe direction of communication in the LVDS circuits as round robin in afirst selected mode and a split in a second selected mode to accommodatethe desired structure and to provide redundancy for failure byreprogramming from round robin to split to communicate around a failedslave DSLAM.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reference to the following detailed descriptionwhen considered in connection with the accompanying drawings wherein:

FIG. 1 a is a prior art example of a single link daisy chain connection;

FIG. 1 b is a prior art example of a star interconnection architecture;

FIG. 2 a is a schematic diagram of an exemplary embodiment of thepresent invention employing a ring structure with a round robin datapath;

FIG. 2 b is a schematic diagram of an exemplary embodiment of thepresent invention employing a split stacking with split data path;

FIG. 2 c is a block diagram of an exemplary embodiment of the inventionfor N=3;

FIG. 2 d is a block diagram of an exemplary embodiment of the inventionfor N=7;

FIG. 3 is a schematic diagram illustrating the traffic redirectioncapability of the redundant structure of the present invention;

FIG. 4 is a schematic diagram of dual master stacking possible with thepresent invention; and,

FIG. 5 is a detailed schematic diagram of the main functional blocks ofthe master and slave DSLAMs in a configuration according to the presentinvention;

FIG. 6 is a schematic demonstration of an embodiment for communicationsbetween DSLAMs employing the present invention:

FIG. 7 shows an example of the relay/stacking I/O table in a SlaveDSLAM;

FIG. 8 illustrates an example of the proxy table in the Master for theembodiment of FIG. 2 d with N=7;

FIG. 9 illustrates the communications relaying scheme for the Slaves'downlink path;

FIG. 10 illustrates the communications relaying scheme for the Slaves'uplink path;

FIG. 11 is a flow chart depicting ATM uplink communications in anembodiment of the invention;

FIG. 12 is is a flow chart depicting ATM downlink communications in anembodiment of the invention; and,

FIG. 13 is a state diagram for in-band communications in an embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

The proposed method improves the conventional stacking approach byproviding dual links between the master and slave boxes in a “ring”structure, such that redundancy is built-in and software controllable.Further, the data path route from master to slave is also softwareconfigurable. FIGS. 2 a and b show the proposed new method of stackingbased on “ring” or “dual link” architecture respectively. The stackinglink 10 connecting the master box 12 and slave boxes 14 for theembodiments shown is based on low voltage differential signaling (LVDS)technology, designed to carry ATM traffic, as will be discussed ingreater detail subsequently. The concept, however, can apply for othertypes of links. For illustration purposes, the uplink 16 for datatraffic is shown as ATM based connecting to the central office (CO) orATM network via OC-3 or (Multilink) E1/T1 IMA. In the “ring” structurewith “round robin” data communication configuration shown in FIG. 2 a,the data flows in one direction from one box to another (i.e. to the onestacked below). The last box down the chain completes the “ring” byfollowing the same data path to the master. In the “dual link” structurewith “split path” data communication configuration, the data flowsbi-directionally (transmit/receive) from one box to another (i.e. to theone stacked below). The master splits the two traffic paths via softwarecontrol, based either on virtual slave box identification numbers and/orATM virtual path/virtual circuit numbers of a slave box.

The configuration described is further illustrated in FIGS. 2 c and 2 dfor N=3 and N=7 respectively. Here, the Master to Slave and Slave toSlave interconnections alternate in each link, as shown. In one link,for example, the chain connection order is from Master to Slave 1 toSlave 3, etc. In the other link, the chain order is from Master to Slave2 to Slave 4, etc. The last two slave boxes in each chain link thenconnect with each other on link 10′ to complete the loop for the purposeof redundancy and traffic redirection in case a failure occurs as willbe described in greater detail subsequently.

An additional alternative or simple dual link connection will be thesame as shown in FIG. 2 a, but the traffic path is reprogrammed by theMaster to be bi-directional on each link. The two paths will be, for anexample of N=7, (a) Master to Slave 1, 2, 3 and (b) Master to Slave 7,6, 5, and 4. In this case, the link interconnection between Slave 3 andSlave 4 will not be used for data traffic but for redundancy.

A third alternative provides the two slave boxes connecting to theMaster are the first Slave and a middle slave. In case of N=3, themiddle slave will #2. One link connects Master and Slave 1, and theother link connects Master and Slave 2 and then Slave 3. For N=7, themiddle slave will be 4 and one link will be Master and Slave 1 and then2 and 3. The other link will be Master and Slave 4, and then 5 to 7.Again, the loop closes by linking together Slaves 1 and 3 for N=3 orSlaves 3 and 7 for N=7, for the purpose of redundancy. Note that withthis configuration, the longest delay between master and slave issubstantially half of that with the “ring” configuration or single linkdaisy chain.

The flexibility of the invention allows this configuration to also beused for a Round Robin daisy chain link since the virtual “position” ofthe slave DSLAMs is programmable. The data path can be programmed at theinitialization phase to be uni-directional on the link, i.e., in theorder of Master to Slave 1 to Slave 3 to Slave 2 to Master for N=3, andMaster to Slave 1 to Slave 3 to Slave 5 to Slave 7 to Slave 6 to Slave 4to Slave 2 to Master for N=7. In this case, the last link (10′) willalso be used for data traffic. The link path can be reconfigured by theMaster to bi-directional as shown in FIG. 2 d when a failure occurs.

FIG. 3 shows an example of the reliability feature or self-healingavailable with the method of the present invention. As shown for a ringstack chain physical configuration, one slave box 14′ going down willnot affect the operation of the rest of the stacked boxes. The mastercan redirect traffic when informed the failure has occurred in one ofthe slave boxes by converting the traffic in the slave boxes to splitpath with bi-direction communication on both sides of the “down” slavebox with communication to the master box at both ends of the chain. Forthe configurations described above with respect to FIGS. 2 c and 2 d,the link 10′ is activated for normal communication.

Detecting a failure/malfunction of a slave DSLAM is accomplished by theMaster for the embodiments shown in one of several approaches. A firstapproach is for Master to periodically send a “heart-beat” message toeach slave by means of polling in a round robin fashion. If the slavebeing polled (identified by virtual ID) does not respond within apredesignated time, the Master will consider it has failed and redirecttraffic around that slave.

A second approach is that the slaves communicate with each other bysending heart-beat messages. As an example, the architecture can beprogrammed by the Master such that the box that is stacked “on top”always sends messages to the one that is stacked “below” or next incommunication sequence. In the round-robin chain case, the Master sendsheart-beat to Slave 1, Slave 1 sends the heart-beat to Slave 2. When thebox receiving the heart-beat message does not respond within certaintime, the box sending the message will consider that box being failedand inform the Master using the reserved in-band communication channel(as described below).

FIG. 4 shows an embodiment of the present invention with a dual masterconfiguration with a second master box 18, providing full protection inthe event one master fails. Coordination between the two masters isemployed, in terms of traffic and configuration, such as active/standby,shared active/active, etc.

FIG. 5 shows the internal main functional architecture of the master andslave DSLAMs and a general stacking arrangement. The master boxincorporates multiple ADSL interfaces 20 for connection to the userlines 22. A control circuit 24 with a central processing unit (CPU) andATM switching and traffic management controls the master DSLAM. Anuplink circuit 26 provides communication with the CO using STM/OC3 ormultilink EI/TI IMA. Communication with the slave boxes is provided bymultiple LVDS circuits 28 a and 28 b.

Each slave box also has multiple ADSL interfaces 30 for connection tothe user lines 32. A control circuit 34 with a central processing unit(CPU) and ATM switching and traffic management controls the slave DSLAMunder software control by the master. LVDS ports 36 a and 36 binterconnect the slave box for communication with each box adjacent inthe circuit connection, either master or slave. The control circuit inthe master DSLAM provides software instruction to the slave DSLAMs forestablishing the communication direction as round robin or split, i.e.bi-directional, as will be described in greater detail subsequently.With the physical connection in a ring stack chain, the normalcommunication path would be round robin. However, failure of one slavebox can be overcome by the master reprogramming the slaves tocommunicate in split format thereby automatically reconfiguring thecircuit arrangement to a dual link configuration as previously describedwith respect to FIG. 3.

The communications between the Master and Slaves and between the SlaveDSLAMs for the embodiments shown are based on in-band communicationscarried within the LVDS link and ATM cell streams. This is also the CPUto CPU communication means between two DSLAM boxes via an in-band LVDSchannel. FIG. 6 demonstrates this embodiment in schematic form. The CPUmessages are inserted into the ATM cell streams by the LVDS circuitthrough the CPU interface as represented by links 40. The in-bandcommunications are transmitted by the LVDS with the ATM cell streams onlinks 10 between DSLAMs.

Communication between the master and slave boxes employs a proxy and/orrelaying traffic mechanism/scheme for data and management messages,based on virtual box identification and virtual path/circuitidentifications. Each Slave DSLAM maintains a traffic relay and stackingI/O table that does the virtual channel and box ID mapping. The table isupdated whenever instructed by the Master via in-band communicationchannel. FIG. 7 shows an example of the relay/stacking I/O table in aSlave DSLAM. The table incorporates the ADSL port numbers connected tothe DSLAM, the VCI/VPIs for the Slave DSLAM, the virtual ID for theDSLAM and the Stacking Input/Output mode for each of the two LVDS ports.The stacking ports in the Utopia LVDS configuration shown for theembodiments in the drawings can be predefined as belonging to the “A” or“B” LVDS link during initialization. Each port can be programmed forinput, output or bi-directional (input and output) communication.

The Master DSLAM maintains a “proxy” table for all slaves, whichincludes VCI/VPI cross-connect information and mapping between externalVCI/VPI to internal (slave) VCI/VPI, as well as stacking I/Oconfiguration. FIG. 8 illustrates an example of the proxy table in theMaster for the embodiment of FIG. 2 d with N=7. The proxy table containsthe information present in the tables for each of the Slaves and anadditional data item defining the Master Stacking Mode for round robinor split path. The Stacking Input/Output mode established by the Masterwill be determined by the Stacking mode as previously described. Failuremode recovery can be accomplished by the Master as previously describedresulting in a change in the Stacking mode and bypassing of a failedSlave DSLAM in the communications path.

The communications relaying scheme for the Slaves is illustrated inFIGS. 9 and 10 for downlink and uplink paths respectively. Referring toFIG. 9, the ATM data from the uplink is received by a DSLAM in block 50and routed through an ATM switch 52. A determination is made in block 54comparing the ATM cells with the DSLAM traffic relay and stacking tableto determine if the communication is local or should be downlinked inthe stack. If the communication is intended for an ADSL port in theDSLAM, it is switched to the appropriate local ADSL port in block 56. Ifthe communication is to be passed through, a check of the stacking I/Otable is made to determine the transfer port in block 58. If the I/Otable has been updated, the communication is passed to the new stackingport in block 60 for ATM output to the next DSLAM as defined by the newstacking port data. If the I/O table has not been updated, thecommunication is passed to the old stacking port in block 62 for ATMoutput to the next DSLAM as defined by the old stacking port data.

Uplinking of data is shown in FIG. 10 where ATM data from the Downlinkis received in block 64 from the local ADSL ports in the DSLAM or fromthe stacking input port. The data is routed through the ATM switch 66and to the uplink switch 68 for external communication. A check of thestacking I/O table is made to determine the transfer port in block 70.If the I/O table has not been updated, the communication is passed tothe old stacking port in block 72 for ATM output to the next DSLAM asdefined by the old stacking port data. If the I/O table has beenupdated, the communication is passed to the new stacking port in block74 for ATM output to the next DSLAM as defined by the new stacking portdata.

Uplink and downlink communications for the Master DSLAM are conductedbased on the proxy table previously described with respect to FIG. 8. Asshown in FIG. 11, ATM data is received by the Master from the uplink inblock 80 through the ATM switch 82. A determination 84 is made whetherthe ATM cells correspond to a local ADSL port for the Master or requirea stack downlink. If local the ATM cells are switched to the appropriatelocal DSL port in block 86, if not, a check 88 is made to determine thestacking mode. If a determination is made in block 90 that the roundrobin mode is present, the Master switches to the output stacking portpreviously defined in block 92. The proxy table allows conversion of theexternal VCI/VPI to internal (slave) VCI/VPI for downlink. If a splitmode is used, a switch to the proper stacking port is made using theVCI/VPI cross connect table in block 94.

The uplink proxy path is shown in FIG. 12. ATM data from the local ADSLport in the Master or downlink slave DSLAM is received in block 96through the ATM switch 98 and transmitted through the uplink in switch100 with the Master conducting the VCI/VPI cross connect as requiredpursuant to the proxy table.

Heartbeat communications and failure reprogramming are accomplished bythe Master DSLAM using inband messaging as previously described. For thecase of a Round-Robin chain link in the embodiments shown, the failuresensing schemes reserve the in-band communication channel that is in thereverse direction of the normal ATM traffic, in order to inform theMaster when failure occurs. For the case of split-path chain link,however, since each link is already bi-directional, the in-bandcommunication channel is carried over the normal ATM traffic.

The Master reprograms the Slave path direction in case of failurethrough the in-band communication channel by sending down stacking pathI/O messages to each of the Slave boxes (minus the failed one). TheSlave boxes then update their corresponding I/O tables. The Masterincorporates a simple state machine that tracks the operating conditionof each Slave box in its stacking control circuit.

Similarly, when the failed DSLAM box is recovered to be operationalagain, it will inform the Master through the in-band communicationchannel to notify that it is alive. The Master then will either updatethe I/O table of all the Slaves including the recovered one, or byupdating only the recovered Slave of its normal traffic I/O path thatfollows the rest of the traffic directions.

The state machine for the described in-band communications is shown inFIG. 13. At start-up, the Master sends an initialization communication102 to define the stacking I/O information table for each DSLAM. As longas all Slaves remain operational, state 104, in response to heartbeatsignals, communications proceed based on the initialized settings. If noheart-beat response is received or a failure notification is received,the Master enters state 106, identifies the failed Slave by its virtualID and sends stacking I/O table updates to the remaining operationalslaves. The remaining Slaves continue operation with the revised I/Ostacking tables until the failed Slave communicates with the Master thatit is recovered. Upon such notification, the Master enters state 108 andagain sends updated stacking I/O table information to all Slaves in thestack before returning to the Operational state 104.

Having now described the invention in detail as required by the patentstatutes, those skilled in the art will recognize modifications andsubstitutions to the specific embodiments disclosed herein. Suchmodifications are within the scope and intent of the present inventionas defined in the following claims.

1. An architecture for stacking Digital Subscriber Line AccessMultiplexers (DSLAMs) comprising: a single master DSLAM having a controlcircuit and an uplink communication circuit to a central office and twolow voltage differential signaling (LVDS) circuits for communicationwith two DSLAM boxes in direct adjacent circuit interconnection to thesingle master DSLAM; a plurality of slave DSLAMs each having aprogrammable control circuit and two LVDS circuits for communicationwith two DSLAM boxes in adjacent virtual circuit interconnection; andmeans for selectively programming the control circuits in each slaveDSLAM to control the direction of communication in the LVDS circuits insaid each slave DSLAM as round robin in a first selected mode and asplit in a second selected mode, said split selectively programmed at adesired virtual circuit connection adjacent to the single master DSLAMfor communication only through that single master DSLAM.
 2. Anarchitecture for stacking DSLAMs as defined in claim 1 wherein the twoDSLAM boxes in adjacent circuit connection to the single master DSLAMare a first and a last slave DSLAMs to form a ring structure.
 3. Anarchitecture for stacking DSLAMs as defined in claim 1 wherein the twoDSLAM boxes in direct adjacent circuit connection to the master are afirst and a middle slave DSLAMs to form a dual link structure.
 4. Anarchitecture for stacking DSLAMs as defined in claim 1 wherein the meansfor selectively programming the slave DSLAMs employs a proxy scheme fordata and management messages, based on virtual box identification andvirtual path/circuit identifications.
 5. An architecture for stackingDSLAMs as defined in claim 1 wherein the means for selectivelyprogramming the slave DSLAMs employs a relaying traffic mechanism fordata and management messages, based on virtual box identification andvirtual path/circuit identifications.
 6. An architecture for stackingDSLAMs as defined in claim 1 further comprising means for detectingfailure of one of the slave DSLAMs and wherein the means for selectivelyprogramming is responsive to the means for detecting failure to allowreprogramming from a round robin to split path communication around thefailed slave DSLAM while retaining communication only through the singlemaster DSLAM.
 7. An architecture for stacking DSLAMs as defined in claim6 further comprising a standby master wherein upon failure of the singlemaster DSLAM, the standby master is enabled for operation as a singlemaster having a control circuit and an uplink communication circuit to acentral office and two low voltage differential signaling (LVDS)circuits for communication with two DSLAM boxes in direct adjacentcircuit interconnection to the single master DSLAM and said means forselectively programming reprograms from a round robin to split pathcommunication around the failed single master DSLAM while retainingcommunication through the standby master as a new single master DSLAMsaid split selectively programmed at a desired virtual circuitconnection adjacent to the standby master for communication through thestandby master.
 8. An architecture for stacking Digital Subscriber LineAccess Multiplexers (DSLAMs) comprising: a single master DSLAM having acontrol circuit and an uplink communication circuit to a central officeand two low voltage differential signaling (LVDS) circuits forcommunication with two DSLAM boxes in direct adjacent circuitinterconnection to the single master DSLAM; a plurality of slave DSLAMseach having a programmable control circuit and two LVDS circuits forcommunication with two DSLAM boxes in adjacent virtual circuitinterconnection; and means for selectively programming the controlcircuits in each slave DSLAM to control the direction of communicationin the LVDS circuits in said each slave DSLAM as round robin in a firstselected mode and a split in a second selected mode, said splitselectively programmed at a desired virtual circuit connection adjacentto the single master for communication through that single master DSLAM;means for detecting failure of one of the slave DSLAMs and wherein themeans for selectively programming is responsive to the means fordetecting failure to allow reprogramming from a round robin to splitpath communication around the failed slave DSLAM while retainingcommunication through the single master DSLAM; and a standby masterwherein upon failure of the single master, the standby master is enabledfor operation as a single master having a control circuit and an uplinkcommunication circuit to a central office and two low voltagedifferential signaling (LVDS) circuits for communication with two DSLAMboxes in direct adjacent circuit interconnection to the single masterDSLAM, and said means for selectively programming reprograms from around robin to split path communication around the failed single masterDSLAM while retaining communication through the standby master as a newsingle master DSLAM, said split selectively programmed at a desiredvirtual circuit connection adjacent to the standby master forcommunication through the standby master.